That depends on which property of the water you need to measure.
-- Its temperature . . . degrees Celsius or Fahrenheit.
-- Its volume . . . liter or fluid ounce.
-- Its weight . . . Newton or pound.
-- Its mass . . . kilogram or pound.
-- Its conductivity . . . Siemens.
-- Its depth . . . inch or centimeter.
-- Its clarity or purity . . . I don't know units for either of those, but my only point
is that you don't "measure water" . . . you measure its properties.
I am guessing you can use Gallons.
The unit of measure here would probably be l, for liters.
Which unit would you use to measure the water in a full tub?
Gallons
Litres
I am guessing you can use Gallons.
The unit of measure here would probably be l, for liters.
Liters would be easiest to use.
Litres for a domestic aquarium, kilolitres for a large [public] aquarium.
Measure its dimensions in inches or centimeters, and its volume or capacity in gallons or litres.
Which unit would you use to measure the water in a full tub?
Gallons.
Gallons
If you are trying to measure concentration of salt in water you would be measuring molarity which is moles / liter. Moles are equal to grams of salt / the MW of salt (and if you mean NaCl then the MW is 58.44 g/mol). Density is a ratio of mass to volume (mass / volume or kg/m^3)
You would use the metric unit "liters" to measure the capacity of a hot water tank.
Litre
gallons